Aluminum alloy



April 21, 1942. P 'T sTRQ-UP 2,280,169

ALUMINUM ALLOY Original Fi led Oct 27, 1939 INVENTOR Phi/ ,0 7." Sfraup ATTORNEY num base alloys does not adversely afl'ect other Patented Apr. 21, 1942 UNITED STATES PAT ENT OFFICE I ALUMINUM ALLOY Philip '1. Stroup, New Kensington, Pa., assignor to Aluini Pa.,

num Company of America, Pittsburgh, a corporation of Pennsylvania Original application October 27, 1939, Serial No; 301,594. Divided and this application October 1, 1940, Serial No. 359,239-

1 Claim. (crews-13s) I This invention relates to aluminum base alloys, andit is particularly concerned with controlling the grain size in castings. This is a divisional application of my copending application, Serial No. 301,594, filed October 27, 1939.

--Among the factors which afiect the properties and behavior of both wrought and cast aluminum base alloy articles, one of the most important is the grain size of the metal. The term grain size refers to the dimensions of the individual crystals which compose the metallic body. -The grain size is usually referred to as being fine, medium, or coarse, and the shape of the grains is described as beingequi-axed or elongated, depending upon the relative dimensions of the grain. Generally, a'flne equi-axed grain size is considered to be most desirable in an alloy both from the standpoint of strength and hardness, as well as workability. Since some aluminum base alloys do not inherently exhibit a small grain size inthe as-cast condition, and, furthermore, since thermal conditions during solidification of the molten metal exercise such a great and (3) a minimum of undesired effect on other important properties.

7 It is the principal object of my invention to provide a simple means for producing small equiaxed grains in cast aluminum base alloys. An-

other object is to provide a means for effecting this control of grain size which has the abovementioned characteristics. These and other objects will become apparent from the following description of my invention.

I have discovered that the addition of small properties which are generally desired, such as hardness, strength, ductility, workability, and resistance to corrosion. I have also observed that the grain-refining eifect obtained through the addition of these elements is substantially uniform throughoutthe entire article. This uniformity in effect is particularly advantageous in from the cast article. By emphasizing the effect influence upon the sizeof grains, it is necessary' upon the grain size of the cast alloys, I do not wish to minimize any advantages gained in other respects.

Only relatively small amounts of columbium and tantalum are required to produce a fine grain size in castings, .from 0.01 to 0.1 per cent of either one generally being suflicient for the purpose. In certainoases it may be'necessary to employ even more, but in no event should the amount exceed 0.5 per cent, and preferably not. over 0.4 per cent. Although either elementis eflective when used separately, I have found that an even more pronounced grain-refining effect is obtained if both are simultaneously employed. In such a case the total'amount should not be less than about 0.02 it exceed about 0.5 per cent.

The elements columbium and tantalum, for the purposes of my invention, are regarded as being equivalent to each other, that is, one may be substituted for the other although not necesper cent, nor should sarily in the same proportions, and therefore produces a small grain size in the as-cast prodv uct. While the presence of either element alone in an alloy has a pronounced eiiect upon the grain size, an even greater eflect is obtained if 4 both elements are present. As far as I have observed, the addition oi these elements to alumithey constitute a group. In addition to having I a similar grain refining effect on aluminum base alloys, these elements resemble each other in that both of them occur in the same subgroup of group V of the periodic table, both have bodycentered space lattices, and both form the same type of alloy constitutional diagram with aluminum.

The aluminum base alloys which are particularly benefited by the addition of at least one of the elements of the columbium-tantalum group are those containing .from 0.25 to 12 per cent copper, or 0.5 to 15 per cent magnesium,

, or 0.25 to 14 per cen'tsilicon, or 0.5 to 20 per cent zinc, or 0.1 to' 3 per cent manganese, or combinations or two ormore of these elements. These I per cent chromium,

. alloys may also contain one or more of the following elements, often referred to as "hardeners," in the following percentages: 0.05 to 0.5 0.01 to 0.5 per cent titanium. 0.25 to 2.5 per cent nickel, 0.01 to 0.5 per cent boron, 0.002 to 2 per cent beryllium, 0.1 to 0.5 per cent molybdenum, and 0.1 to 0.5 per cent zirconium. The total amount of the latter elements, however, should not exceed about 3 per cent. As exemplary of the variety of alloys whose grain size has been found to be reduced by the addition of columbium and/or tantalum, the following compositions are cited, wherein aluminum constitutes the balance of the alloy in each case: 1.25 per cent manganese; 2.5 per cent magnesium, 0.25 per cent chromium; 2 per cent MgaSi, 0.25 per cent chromium; 4 per cent copper; 5 per cent silicon; 5.25 per cent MgZm; 1.25 per cent magnesium, 0.5 per cent zinc, 0.15 per cent copper; and 4.4 per cent copper, 0.65 per cent manganese, 1.5 per cent magnesium.

The effect of adding columbium or tantalum, or both elements, to a particular alloy'is illustrated in the accompanying iigures,'where Fig. 1 is a photomicrograph of an as-cast alloy composed of 2.5 per cent magnesium, 0.25 per cent chromium, the balance commercially pure aluminum;

Fig. 2 is a photomicrograph of the same alloy to which 0.03 per cent columbium had been added;

Fig. 3 is a photomicrograph of the same alloy to which 0.06 per cent tantalum had been added;

and

Fig. 4 is a photomicrograph of the same alloy to which 0.02 per cent columbium and 0.07 per cent tantalum had been added.

The alloy employed for the test was one which is widely used in wrought form, and has a nominal composition of 2.5 per cent magnesium, 0.25 per cent chromium, and the balance aluminum containing a maximum of 0.3 per cent iron and silicon as impurities. A quantity of the alloy was first melted and a specimen poured at a tem-' perature of 1350 F. into a cold, thin-walled iron mold having the shape of a frustum of an inverted cone with a diameter of about three inches at the base of the cone. About five minutes was required for the metal to completely solidify, which tended to promote the formation of large grains. The remaining melt was divided into three portions, 0.03 per cent columbium being In Fig. 1 the large grains of the untreated alloy may be plainly seen. Grains of this size are regarded as being too coarse. for a satisfactory casting as well as promoting cracking and checking in a body that is to be subsequently worked. The criss-cross markings on some of the grains illustrate a common solidification phenomenon known as dendritic formation. The grainrefining efiect of adding columbium to the alloy is seen in Fig. 2. In comparison to Fig. 1, the grains are very small and equi-axed. In Fig. 3, the grain size of the alloy to which tantalum had been added may be seen. Since 0.06 per cent tantalum was employed, as compared to 0.03 per cent columbium in the preceding example, it is not surprising that the grain size shouldbe smaller than in Fig. 2. The very marked eiiect of 'both columbium and tantalum on the grain size is shown in Fig. 4. The grain size is so small as to be scarcely distinguishable at a magnification of three diameters, which is the same magnification that was used in the other photomicrographs.

The tantalum and columbium may be added to molten aluminum base alloys in any convenient manner. I have found that the ferro-alloys of these two elements provide a satisfactory source. The ferro-alloy is preferably diluted with aluminum at a high temperature, andthis diluted alloy containing, for example, 2 to 5 per cent of .columbium or tantalum is used for making additions. This diluted alloy may be referred to as ahardening or rich alloy. Generally speaking, since the amount of tantalum and columbium used is so small, the amount of iron which is also introduced along with these elements from the ferro-alloys is likewise small and has no significant effect in the case of most alloys. Another advantage obtained 1 through using the ferro-alloys as a source of columbium and tantalum is thatboth of these elements will usually be present and therefore tend to produce an even added to one, 0.06 per cent tantalum being added to the second, and 0.02 per cent columbium and 0.07 per cent tantalum being added to the third. Specimens were cast at atemperature of 1350" F. in the same iron mold as the alloy without the columbium or tantalum additions, the mold in each case being at room temperature, or cold, when the metal was poured into it. The specimens were sectioned in a vertical plane, polished, and etched in an aqueous solution of nitric and hydrochloric acids. A representative finer structure than if only one is employed.

In referring to aluminum base alloys herein, I mean those which contain at least 50 per cent aluminum. The term aluminum as herein employed refers to the metal as commercially produced which contains impurities.

Where, in the appended claim, the balance of an alloy is said to be substantially aluminum," it is intended that this expression shall permit the inclusion in the alloy composition of one or more of the hardening elements mentioned hereina-bove as well as the usual impurities.

section of each specimen was then photographed V at a magnification of three diameters.

I claim:

A cast article composed of an aluminum base alloy containing from 0.1 to 3 per cent manganese, and at least 0.01 per cent of each of the metals tantalum and columbium, the total amount of said two metals not exceeding 0.5 per cent, the balance being aluminum, said alloy having a finer grain size in the as-cast condition than the same alloy containing either tantalum or columbium alone.

PHILIP T. STROUP. 

